1. Field of the Invention
This application relates to a fluid flow measuring apparatus, and more particularly to a self-compensating optical drop count apparatus for measuring volumetric fluid flow of low pressure fluids.
2. Description of the Prior Art
The need for an accurate in-line volumetric fluid flow measurement apparatus has long existed. When fluid is being transferred from one storage area to another through a tube, pipe, or similar line, it is useful to know exactly how much fluid has been transferred without the need of removing the transferred fluid from the line and measuring it in containers of known volume.
In-line fluid flow measurement devices usually require that the fluid being measured by under sufficient pressure to activate some sort of monitoring device, such as a paddle wheel, that is inserted in the line. Such monitoring devices not only require that the fluid be under pressure, but they also tend to impede the normal fluid flow. Such devices are thus ill suited for low pressure fluid flow applications.
Recent advances in medical technology have demonstrated a critical need for accurate real time measurements of body fluids for patients in hospitals and other health care facilities. Such fluids, which are usually under very low pressure, may be infused into the patient or excreted from the patient. Moreover, the fluids rate is typically drops of fluid rather than a constant stream. Also, because of the medical environment involved, only a closed measurement system can be used--i.e., one that does not violate the integrity of protection against bacterial infection.
Prior art systems for measuring body fluid flow have consisted primarily of monitoring the decrease in volume of a supply of fluid, in the case of fluids being infused into the body (such as blood or plasma), over a period of time. Correspondingly, in the case of fluids being excreted from the body (such as urine), fluid flow is typically measured by monitoring the increase in volume over a period of time in a reservoir, or other type of collection bag, where the excreted fluids are deposited. Other prior art systems weigh the fluid over a period of time. In all cases, the measurement is not a "real-time" measurement in the sense that a significant time period must usually pass before a meaningful measurement can be made.
With the advent of optical electronics, attempts have been made to measure the volume of fluid flow by optically sensing and counting the number of drops that pass through a drop chamber. When these drops of fluid are relatively uniform in volume, volumetric fluid flow is, in theory, easily derived from the drop count by merely multiplying the drop count by the known average volume per drop. However, numerous problems have been encountered with this approach. For one, the volume of fluid in each drop is not necessarily uniform, but may vary with the drop rate, a fast drop rate producing drops with more fluid volume than a slow drop rate. Also, there have been problems with maintaining the integrity of the light beam that must pass through the drop chamber where the drops are falling. Splashing of the fluid after the drops have fallen, as well as condensation that occurs within the drop chamber, or foreign matter on the outside off the chamber, tend to block the light beam, or reduce its intensity, so that it can no longer accurately sense the drops passing through the chamber. Also, the drop may break up into two or more drops as it falls, thereby causing a false count condition to exist.